Strength of optical quality polycrystalline CVD diamond

The three-point loading method is used to the measure the fracture strength σf of polycrystalline CVD diamond plates with thickness in the range of h = 0.06–1.0 mm. Optical quality samples grown in a microwave plasma using CH4–H2 gas mixtures show an inherently nonuniform structure, the crystallite size varying (increasing) by 1–2 orders of magnitude in traversing from the substrate side to growth side. The value of σf approaches ≈ 2200 MPa for the thinnest film when the fine-grained (substrate) side is under tensile stress, reducing with plate thickness down to ≈ 600 MPa at h ≈1000 μm. The strength is approximately a factor of two lower for the substrate side under tensile stress. In general, the material tested follows Hall–Petch relationship—a stress increase with grain size reduction. The fracture statistics are analyzed using a Weibull distribution, and a Weibull modulus m of 6.4 and 4.5 is found for the growth and substrate side under tension, respectively. Young’s modulus E = 1072 ± 153 GPa for polycrystalline diamond is evaluated from the same tests. © Pleiades Publishing, Ltd., 2011.